The present invention relates generally to ball bearings for linear motion and to bearing shafts having ball conforming grooves and relates more particularly to certain new and useful improvements in such ball bearings and cooperating bearing shafts and to new and improved methods for providing the bearing shafts with such ball conforming grooves.
With a few exceptions linear motion ball bearings are run on cylindrical shafts having a smooth, ground, uninterrupted surface. The mating contact between the load carrying balls and the shaft surface is between two convex shapes. Theoretically, this results in a contact area of "zero." In practice, due to deflection of materials, a very small area of contact is developed. This is generally referred to as a "point contact."
In the sleeve of a linear ball bearing, the load carrying balls bear on a concave surface. The contact of this concave surface with the convex surface of the ball creates a line contact, rather than a point contact, which greatly reduces stresses developed when compared to the point contact of the ball with the shaft. For this reason, the critical point in the bearing and shaft assembly is the contact of the ball with the shaft, because of the severe stresses created in both the ball and the shaft. To alleviate this condition, attempts have been made to provide ball conforming grooves in the shaft upon which a linear motion ball bearing is mounted in an effort to provide line contact instead of a point contact. This effort has not been entirely successful because of the cost of grinding, or otherwise forming, the longitudinal grooves along the length of the shafting and the fact that it is not commercially feasible to maintain an exact angular position around the circumference of the shaft along its length.
It is desirable that the shafts upon which linear motion ball bearing assemblies freely move have hardened surfaces to carry the load transferred by the balls and to effectively utilize the load carrying capacity of such bearing assemblies. In the past, shafts for linear motion ball bearings, whether provided with smooth surfaces or with longitudinal grooves, have been heat treated to approximately 60 Rockwell "C" hardness and subsequently ground to the desired size and finish. These procedures when practiced on a commercial basis are costly due to the warpage resulting from unequal stress distribution in the shaft attributable to the heat treating. Straightening of the shaft to remove the warpage has been required before the grinding and finishing steps have been performed. Frequently, it may be required to subject the shaft to straightening between passes through a grinding machine as the shaft is ground down to the final finished size. Such efforts to straighten the shaft, when required, are tedious and costly. Even where such a shaft is subjected to straightening, true alignment of a hardened metal shaft may not be achieved as commercially available straightening machines might not effectively correct small deviations from a true alignment, because the hardened metal of the shaft has unequal stress distribution and resists the force applied to straighten the shaft.
On the other hand, non-hardened steel shafts are less costly than hardened steel shafts, and may be more readily straightened and given true alignment. The use of non-hardened shafts would serve to eliminate warpage due to heat treating and the attendant manufacturing costs which otherwise would be incurred in attempting to straighten a hardened shaft. A non-hardened shaft, however, is not ideally suited for use with a linear motion ball bearing because it might not carry the load transferred by the balls and would not effectively utilize the load carrying capacity of the linear motion ball bearing.
This invention is directed to improvements in a linear motion ball bearing and in shafting upon which such bearing may move. The shafts in accordance with this invention may be formed of a low cost, soft, non-hardened or non-heat treatable material having one or more ball conforming groove, the grooves being formed either directly in the surface of the shaft or in strips inserted into the surface of the shaft or in strips placed on and secured to the surface of the shaft. According to one aspect of the invention, a non-hardened shaft, or non-hardened strips located on the shaft may be rolled to form and work harden a ball conforming groove in the unhardened shaft surface or the unhardened strip before or after it is located on the shaft. Thus, this invention may advantageously utilize non-hardened metal shafting which can be given true alignment and in or upon which there may be formed or placed hardened or hardenable portions for effectively carrying the load. The bearing is so constructed as to enable it to adjust its position in very slight amounts, if necessary, to compensate for manufacturing tolerances in the formation of the ball conforming grooves located on the shaft so that the ball conforming grooves formed in the sleeve of the linear ball bearing and the ball conforming grooves associated with the shaft are in exact register with the ball thereby serving to eliminate severe stresses which might ordinarily result due to manufacturing variations in parallelism or angular displacement of the ball conforming grooves which has, in the past, led to failure of the parts. This invention also enables significant savings in manufacturing costs, increased life and load capacity of the linear motion ball bearing and shaft and enhances the torque resistance between the bearing and the shaft.
It is therefore an object of this invention to provide a new and improved linear motion ball bearing and bearing shaft.
Another object of this invention is to provide a new and improved linear motion ball bearing and bearing shaft combination having increased load-carrying capacity.
A further object of this invention is to provide a new and improved linear motion ball bearing having load-carrying plates which may move circumferentially a very slight amount and which cooperate with ball conforming grooves located on the bearing shaft providing exact register of the load-carrying plate of the bearing and the grooves located on the bearing shaft and, hence, increased load-carrying capacity.
Another object of this invention is to provide a new and improved bearing shaft.
A further object of this invention is to provide a new and improved bearing shaft which includes one or more ball conforming grooves.
A further object of this invention is to provide a new and improved bearing shaft which may be formed in a true axial line and which has load-bearing portions comprised of hardened metal.
Another object of this invention is to provide a new and improved bearing shaft having load-bearing portions comprised of hardened metal, which may be manufactured at reduced cost over present hardened metal shafts.
Another object of this invention is to provide a new and improved bearing shaft comprised of soft, non-heat-treated material having one or more hardened metal strip members secured thereto, each of which is formed with a ball conforming groove on its outer surface.
Still another object of this invention is to provide a new and improved bearing shaft of soft, non-heat-treated material having one or more hardened ball conforming grooves which have been formed as by rolling and hardened by cold working, the grooves being formed either in the shaft or in non-hardened strips located on the shaft.
Another object of this invention is to provide a new and improved bearing shaft comprised of soft, non-heat-treated material having one or more hardened metal strip members secured thereto, each of which is formed with a ball conforming groove, and in which the strip members are compliant circumferentially of the bearing shaft.
A still further object of this invention is to provide certain new and improved methods for forming ball conforming grooves on bearing shafts.
Objects and advantages of the invention are set forth in part herein and in part will be obvious herefrom, or may be learned by practice with the invention, the same being realized and attained by means of the instrumentalities and combinations pointed out in the appended claims.
The invention consists in the novel parts, constructions, arrangements, combinations, steps, processes and improvements herein shown and described.